OK, I tried it on a different website and it looks completely different. So there's something wrong with that other site.

So, we already had a tax rate and class set up. The class is plain old "Taxable Goods", and all of our products are set to that. The ID is 1. Does that all count as the TIC ID etc?

Lastly, it doesn't like something about our address. A big red box is across the top of the checkout screens that says:
Error Error encountered while verifying address 10 Stage Door Road 12590 Authorization failure. Perhaps username and/or password is incorrect.
If I just keep going, when I try to finalize the order the page goes away and I get the following:

Array ( [0] => A matching lookup could not be found for this authorization )
Our location is a bit odd: physically we're in one town, but our area is assigned to the PO in the next town. So our street address / zip probably don't match.